The electron affinities of the elements from aluminum to chlorine are \(-44,-120,-74,-200.4,\) and \(-384.7 \mathrm{kJ} / \mathrm{mol},\) respectively. Rationalize the trend in these values.

Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negative ion. An element with a more negative electron affinity value indicates that it is more favorable for the atom to gain an electron.

To better understand the trend, let's identify the elements from aluminum to chlorine on the periodic table: - Aluminum (Al, atomic number 13) - Silicon (Si, atomic number 14) - Phosphorus (P, atomic number 15) - Sulfur (S, atomic number 16) - Chlorine (Cl, atomic number 17) These elements are in period 3 of the periodic table and belong to groups 13, 14, 15, 16, and 17, respectively.

Now we'll look at the electron affinities in the given order and compare them: - Aluminum (Al): -44 kJ/mol - Silicon (Si): -120 kJ/mol - Phosphorus (P): -74 kJ/mol - Sulfur (S): -200.4 kJ/mol - Chlorine (Cl): -384.7 kJ/mol We can observe the following trend: - The electron affinity generally increases (becomes more negative) from aluminum to chlorine. - There is an exception between silicon and phosphorus where the electron affinity decreases (becomes less negative).

To rationalize the trend, we need to consider the electron configuration of the elements and their positions in the periodic table: 1. Aluminum and silicon are both located in the p-block but have half-filled (Al) and fully filled (Si) s orbitals. Silicon has a more stable electron configuration, which makes it more favorable for the atom to gain an electron, resulting in a higher electron affinity. 2. The electron affinity decreases from silicon to phosphorus due to the introduction of a new electron into the 3p orbital, which causes electron-electron repulsion. This repulsion makes phosphorus less willing to gain an additional electron, resulting in a lower electron affinity. 3. From phosphorus to chlorine, the increase in the effective nuclear charge (number of protons) is responsible for the increase in electron affinity. The more protons in the nucleus, the stronger the attraction for negatively charged electrons, which makes gaining an electron more favorable and electron affinity more negative. In summary, the electron affinities of the elements from aluminum to chlorine show a general trend of increasing (becoming more negative) due to the increase in effective nuclear charge. The exception between silicon and phosphorus can be explained by the electron-electron repulsion caused by adding a new electron into the same orbital.